Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a positive electrode material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal or the like (such a composite oxide may sometimes be referred to as a “lithium-containing composite oxide” in the present invention) such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, LiNiO2, LiNi0.8Co0.2O2 or LiMn2O4, has been known.
Particularly, a lithium ion secondary battery using LiCoO2 as a cathode active material and using as a negative electrode a lithium alloy, carbon such as graphite or carbon fiber, a silicon alloy comprising a composite containing graphite and silicon, or a tin alloy comprising a composite containing tin and carbon, can obtain a high voltage at a level of 4 V, whereby it has been widely used as a battery having a high energy density.
However, in a case where LiCoO2 is used as the positive electrode material, further improvement has been desired e.g. in the discharge capacity, in the average discharge voltage, in the stability against heat during heating (sometimes referred to simply as a safety in the present invention) and in the capacity density per unit volume of the positive electrode layer (sometimes referred to simply as a volume capacity density in the present invention), and it had a problem of e.g. deterioration in the durability for charge and discharge cycles such as a decrease in the discharge capacity of the battery or swelling by a reaction of the interface of the lithium-containing composite oxide with the electrolytic solution, by carrying out charge and discharge cycles repeatedly.
Further, in high voltage application at a discharge voltage of from 4.4 to 4.6 V using as a negative electrode lithium metal, an alloy, carbon or the like, there are problems such as significant deterioration of the charge and discharge cycle durability and swelling of a battery due to generation of a gas such as carbon dioxide by decomposition of the electrolytic solution. Further, along with spread of multifunctional mobile terminals in recent years, a demand for a higher capacity battery has been increasingly high, and it has been desired to develop a cathode material for high voltage which has a high density, which achieves a higher charge and discharge capacity by high voltage discharge, and which has more excellent charge and discharge cycle durability which is an object to be accomplished at the time of use under high voltage than ever before. Further, for a polymer lithium ion secondary battery effective to reduction in thickness of multifunctional portable terminals, swelling-suppressing performance when charged under high voltage and stored at high temperature is more emphasized. The above high voltage more specifically means conditions where a discharge voltage is applied under 4.4 to 4.6 V vs. Li+/Li.
To achieve such an object, for example, use of a surface modified lithium-containing composite oxide comprising a lithium-containing composite oxide having a composition of LiNi0.81Co0.15Al0.04O2 or LiCoO2 covered with zirconium metal, ZrO2 or the like by a sputtering method has been proposed (Patent Documents 1 and 2).
Further, use of a surface modified lithium-containing composite oxide comprising a lithium-containing composite oxide such as Li1.03Co0.98Al0.01Mg0.01O2 or LiCoO2 covered with aluminum phosphate obtained by mixing the lithium-containing composite oxide with an aluminum compound and a phosphorus compound and subjecting the mixture to heat treatment, has been proposed (Patent Documents 3 and 4).
Further, use of a surface modified lithium-containing composite oxide comprising a core of a lithium-containing composite oxide such as LiCoO2 and a surface-treated layer formed on the core which contains P as a first element and Mg, Al or the like as a second element has been proposed (Patent Document 5). Further, use of a surface modified lithium-containing composite oxide comprising a lithium-containing composite oxide such as LiCo0.98Al0.01Mg0.01O2 and a covering layer which contains phosphorus and manganese, or phosphorus and magnesium, has been proposed (Patent Document 6).